Implement processor for prepayments

[QuanMPhm]

Dependant on #102.

We want to implement a processor class for handling group prepayments. Based on the prepayment scaffolding document, these are the changes I believe should be made

• A new processor, PrepaymentProcessor, subclassing DiscountProcessor, should be made. Placed after every other current processors. This processor will set IS_DISCOUNT_BY_NERC to True
• A new command line argument, --prepayment-dir, will be added to determine a local directory which contains the CSVs needed to process prepayments (the prepaid group debit and credit tables, etc). If this directory is not provided, then the script will default to fetching the prepayment documents from Github.
• Aside from fetching from Github, new functions will be added to have the modified prepayment documents pushed back to Github
• A couple of new columns will be added to all invoices except the Lenovo and Non-billable invoice:
  • Prepaid Group Name - Group Name
  • Prepaid Group Institution - calculate institution from Group Contact Email
  • Prepaid Group Balance - SUM(Credits) - SUM(Debits)
  • Prepaid Group Amount Used - the sum of all project costs for this month
• A new set of unit tests will be added.

@naved001 @knikolla @joachimweyl These are questions I would like your considerations on, more may pop-up as I progress through this issue:

• Do we want to backup the prepayment CSVs (credit and debit tables, etc) after each run of the processing script? Where should we back them up to?

[joachimweyl]

sounds right to me.
As for backups, I assume follow the same rules as the invoice backups?